CD-ROM technology was first developed in 1976 and became, by the early 1980s, a widely accepted media for the distribution of recorded music. The potential application of CD-ROM technology for high capacity, low cost data storage and data distribution for computer systems was quickly recognized. CD-ROMs have since become the standard media for distribution of computer software.
FIG. 1 displays basic physical characteristics of a CD-ROM. A CD-ROM 101 is a transparent polycarbonate disk with a radius of 6 cm 102. At the center of the disk is a spindle hole 103 with a radius of 7.5 millimeters. A manufacturer stores data on a CD-ROM by impressing a pattern of flat areas, called lands, and depressions, called pits, onto the surface of the CD-ROM. The surface is then covered with a reflective metallized film, followed by a protective coat of lacquer. Data is read from the CD-ROM by shining a laser beam onto a rotating CD-ROM. Light from the laser beam is reflected by the lands and dispersed by the pits. The light reflected from the lands is detected by a photodiode detector. The lands and pits are arranged along a single spiral track 104 starting near the center of the disk and spiraling out to the edge of the disk. The single spiral track is approximately 4,500 meters in length and 600 nanometers wide. The adjacent turns of the spiral are about 1.6 microns apart.
FIGS. 2A and 2B display the low-level data formatting of a CD-ROM. The single spiral track of the CD-ROM is divided into data sectors. A standard size CD-ROM that stores 60-minutes of recorded music contains 270,000 data sectors. If the outer 5 millimeters of the disk is used for data storage, the resulting CD-ROM that stores 74 minutes of recorded music contains a total of 333,000 data sectors. The data sectors are arranged sequentially along the spiral track 201 from a starting point near the center of the disk outward towards the edge of the disk. Each sector 202 contains 12 bytes of synchronization information 203, followed by four bytes of header information 204, followed by 2,048 bytes of data 205, with the final 288 bytes used for error correcting codes 206.
FIG. 3 displays the read operation of a CD-ROM drive. The laser beam and photodiode detector are mounted on an assembly 301 that moves along a radial vector 302 of the CD-ROM. The CD-ROM is spun in a clockwise direction 303 by a motor. As the CD-ROM is spun, the photodetector/laser assembly moves radially outwards in order to sequentially access each successive sector arranged along the spiral track. In order to read a set of contiguous sectors starting at some arbitrary point on the surface of the CD-ROM, the photodiode/laser assembly is first moved outward to the approximate radius of the CD-ROM corresponding to the first sector of the set of contiguous sectors. This first operation is called seeking. Then each of the contiguous sectors is read in order as the photodiode/laser assembly follows along the track of the spinning disk outward from that point. This latter process is called data transfer. The total time required to read a number of contiguous sectors from a CD-ROM is called the access time, expressed by the following equation: EQU access time=seek time+data transfer time (1)
FIG. 4 displays the higher-level formatting of the data on a CD-ROM. A CD-ROM contains three data sections. The first is a label section 401 that contains a volume title for the CD-ROM. The second section contains directory information that indicates to the software controlling the CD-ROM drive how the remaining data is laid out on the CD-ROM. The format of the directory information may vary depending on the operating system of the computer accessing the CD-ROM. Directories are usually logically ordered as tree-structured hierarchies. Following the directory information are the data files 403. The data files are analogous to data files traditionally stored on magnetic disks within computer systems. A data file contains a number of bytes of data, and may have further internal formatting, depending on the operating system of the computer for which the data file has been created. Each data file is stored on the CD-ROM in one or more contiguous data sectors. The label, directory and file data sections follow one another and are laid out from the starting point of the track near the center of the CD-ROM outward towards the edge of the CD-ROM. Any unused capacity of the CD-ROM occurs between the last used sector of the CD-ROM and the outer edge of the CD-ROM, forming a band of unused sectors at the outer edge of the CD-ROM.
FIG. 5 displays a polar-coordinate-based mathematical representation of a spinning CD-ROM. A location P 501 on the surface of the CD-ROM is designated by the length r of the radial vector 502 from the center of the CD-ROM to the location P and by the angle .theta. 503 by which radial vector is displaced from an arbitrary reference vector 504. The length of the arc S 505 corresponds to the length of the spiral track from the reference vector 504 to the point P 501. The length of this arc is approximately described by the following equation: EQU S=.theta.r (2)
Differentiating both sides of the above equation with respect to time produces the following equation: EQU dS/dt=d.theta./dt r (3)
The rate of change of the length of the arc S with respect to time, dS/dt, represents the linear velocity of the point P along a track of the spinning CD-ROM. The rate of change of the angle .theta. with respect to time, d.theta./dt, represents the angular velocity of the spinning CD-ROM. Thus, the following formula represents the linear velocity of a point moving on a spinning CD-ROM, V, in terms of the angular velocity of the CD-ROM, .omega., and the length of a radial vector describing the location of the point, r: EQU V=.omega.r (4)
The linear velocity of a point moving on a spinning CD-ROM is thus equal to the angular velocity of the CD-ROM disk times the length of the radial vector from the center of the CD-ROM disk to the point.
Lower-speed CD-ROM drives commonly have variable speed motors that spin the CD-ROM disk at different angular velocities in order to keep the linear velocity of points moving under the photodetector/laser assembly constant. The data transfer rate is obviously directly related to the linear velocity at which lands and pits move under the photodetector/laser assembly. In constant linear velocity ("CLV") CD-ROM drives, the data transfer rate is therefore constant over the entire spiral track of the CD-ROM.
FIG. 6 displays a graph of angular velocity versus radius for a CD-ROM read by a CLV CD-ROM drive. The vertical axis 601 of the graph represents the angular velocity. The horizontal axis 602 represents the length of the radial vector from the center of the CD-ROM to a particular data sector. The curve displayed in the graph shows how a CLV drive varies the angular velocity at which it spins a CD-ROM depending on the radius at which the photodetector/laser assembly is reading sectors from the spiral track. The values are given for a standard 150 KB/sec data transfer rate. The angular velocity varies from about 15 revolutions per second at a radius of 1 cm, 603, to about five revolutions per second at a radius of 6 cm, 604. Thus, a CLV drive spins the disk faster in order to access the innermost sectors of a CD-ROM and spins the CD-ROM slower in order to access the sectors at the outer edge of the CD-ROM. Referring to equation (1) shown above, it can be seen that, because the data transfer time is constant over the entire surface of the CD-ROM for a CLV drive, and because the seek time increases with increasing values of location of a data file, the access time for a file on the CD-ROM is directly proportional to the length of the radial vector describing the location of the file. Thus, files located towards the center of the CD-ROM disk can be accessed significantly faster than files located at the outer edges. It is for this reason that the data organization described in FIG. 4 was initially adopted for CD-ROMs. In this data organization, the frequently accessed directory data is located close to the center of the CD-ROM, where access times are smallest. Space capacity is located toward the outer edge of the CD-ROM, where access times are greatest.
The initial standard for CD-ROMs specifies a data transfer rate of 150 KB per second, or, in other words, the reading of 75 data sectors per second by CD-ROM drivers. In order to decrease access times, manufacturers have increased the data transfer rate of CD-ROM drives in multiples of this initial standard data transfer rate. High end CD-ROM drives are currently capable of transferring data at a maximum rate of 16 times the standard 150 KB per second rate. These are known as 16.times. drives. The previous maximum data transfer rate was 12 times the initial standard. Drives designed to transfer data at this previous maximum rate were known as 12.times. drives. 12.times. and slower drives were all CLV drives. However, as manufacturers have increased the angular velocity at which they spin CD-ROMs in CD-ROM drives in order to achieve 16.times. drives, they have begun to use constant angular velocity ("CAV") drives rather than CLV drives, at least for some outer portion of the CD-ROM. Hybrid CLV/CAV CD-ROM drives are CLV drives from the starting point of the track out to some radial distance and then switch to being CAV drives from that point outward to the edge of the disk. Manufacturers have gone to CAV and hybrid CLV/CAV drives because the acceleration forces incurred when seeking from one part of a rapidly-spinning disk to another while correspondingly changing the angular velocity in a CLV drive have become prohibitive at the higher rotation rate of 16.times. drivers.
As pointed out in the article "16.times. CD-ROM Drives: The Truth," PC Magazine, Jun. 24, 1997, p. 29, hereby incorporated by reference, the access times for CD-ROMs nominally rated at 16.times. may actually be significantly greater than access times for CD-ROM drives nominally rated at 12.times.. A need for a new formatting convention for CD-ROMs to be read in CAV and hybrid CAV/CLV CD-ROM drives has therefore been recognized.